Methods of fabricating grating assisted coupler devices

ABSTRACT

Advantageous methodologies are disclosed for embedding periodic patterns in optical waveguide elements such as optical fibers. Polarization independence in an elongated waist region of an add/drop coupler can be established by measuring polarization characteristics during fusion and elongation, and controlling the elongation to impart a cross-sectional shape, such as a hybrid dumbbell-ellipsoid. Polarization dependence can also be minimized by angular deformation of the elements along the light transmissive axis. To write a pattern, an element having potential photosensitivity is markedly photosensitized in a hydrogen or deuterium environment pressurized to about 1000 to 5000 psi and a scanning UV beam that is transmitted through a photo mask and impinges on the coupler waist as the in-diffused gas is constantly replenished. Dimensional variations in the element which can affect spectral bandwidth are sensed by writing a preliminary test pattern in the element and then locally measuring the spectral properties of the test pattern and adjusting the local levels of background index of refractions so that the modal index of refraction is substantially constant along the length of the pattern. A relatively wide scanning writing beam tracks on a narrower waveguide element despite positional imprecision and temporal shifting by using a feedback signal derived from fluorescence induced in the element. To apodize the grating in accordance with a selected function, a constant power beam is directed through a rotating half-wave plate and divided by a polarizing beam splitter into two beams having oppositely varying DC amplitude characteristics. The two beams are separately varied to produce the desired apodization profile of the grating.

This application is a C-I-P of application Ser. No. 08/703,357 now U.S.Pat. No. 5,805,751, issued Sep. 8, 1998 entitled “Wavelength SelectiveOptical Couplers”, and filed Aug. 26, 1996, application Ser. No.08/738,068, now U.S. Pat. No. 5,875,272, issued Feb. 23, 1999“Wavelength Optical Devices”, filed Oct. 25, 1996. This application alsorelies on U.S. provisional application No. 60/055,157 entitled“Fabrication of Add/Drop Filters for Wavelength Division Multiplexing”,filed on Aug. 4, 1997 for priority.

BACKGROUND OF THE INVENTION

This invention relates to optical wave propagation systems and devicesutilizing electro-optical devices, and more particularly to gratingassisted devices for filtering, coupling and other functions.

Communication systems now increasingly employ optical waveguides(optical fibers) which, because of their high speed, low attenuation andwide bandwidth characteristics, can be used for carrying data, video andvoice signals concurrently. An important extension of thesecommunication systems is the use of wavelength division multiplexing, bywhich a given wavelength band is segmented into separate wavelengths sothat multiple traffic can be carried on a single installed line. Thisextension requires the use of multiplexers and demultiplexers which arecapable of dividing the band into given multiples (such as 4, 8, or 16different wavelengths) which are separate but closely spaced. Addingindividual wavelengths to a wideband signal, and extracting a givenwavelength from a multi-wavelength signal, require wavelength selectivecouplers, and this has led to the development of a number of add/dropfilters, the common terminology now used for devices of this type.

Since wavelength selectivity is inherent in a Bragg grating, workers inthe art have devised a number of grating-assisted devices for adding orextracting a given wavelength with respect to a multi-wavelength signal.Typical optical fibers propagate waves by the use of the light confiningand guiding properties of a central core and a surrounding cladding of alower index of refraction. The wave energy is principally propagated inthe core, and a number of add/drop filters or couplers have beendeveloped using Bragg gratings in the core region of one of a pair ofparallel, closely adjacent or touching fibers. The coupling region iscommonly termed “evanescent” in that a signal propagated along one fibercouples over into the other, as an inherent function of the design. Thewavelength selectivity is established by the embedded grating whichprovides forward or backward transmission of the selected wavelength,depending upon chosen grating, characteristics. For modern communicationsystems, however, this approach has a number of functional and operativelimitations, pertaining to such factors as spectral selectivity,signal-to-noise ratio, grating strength, temperature instability andpolarization sensitivity.

The applications referenced above are based upon a novel theoreticalconcept and practical implementation. A narrow waist region of two fuseddissimilar fibers is defined between pairs of tapered coupling sectionsat each end. At the waist, the merged fibers are formed by elongation ofan optical fiber precursor of generally conventional size and are sodiametrically small that the central core effectively vanishes. The waveenergy is transferred through the merged fiber region in two spatiallyoverlapping, orthogonal modes. Although the propagating energies of themodes overlap, the coupling is essentially non-evanescent except in thepresence of a coupling mechanism such as a diffraction grating. Forexample, a reflective grating written in the waist region redirects onlya selected wavelength of an input signal at the input port to the dropport, while all other wavelengths propagate through the waist sectionwithout reflection to the throughput port. This reflection grating thuscouples light between two optical modes in a non-evanescent manner.Numerous advantages derive from this concept and configuration, but therealization of its full potential is dependent upon other developmentalfactors.

For example, modern applications require that any add/drop filter basedupon this concept be very efficient at routing channels, have a stronggrating which can be selectively and precisely placed at or adjusted toa specific wavelength and yet have a limited bandwidth, be temperatureinsensitive, compact, low cost, and not subject to spurious reflectionsor noise in the chosen wavelength band. Achieving high drop efficiencyand low polarization dependence are particularly important. The problemsof achieving these operative properties while at the same time providinga repeatably producible unit of very small size and high sensitivityhave required much further innovation.

SUMMARY OF THE INVENTION

In accordance with the invention, the optical properties and performanceof a grating assisted asymmetric fused coupler are highly dependent onthe physical characteristics of the coupler waist. Polarizationinsensitivity of the drop wavelength can be achieved, for example, bycontrolling the shape during elongation or by applying a permanent twistto the coupler waist after the grating exposure. Furthermore, the smalldiameter waist renders the coupler sensitive to diameternon-uniformities but it is shown that these dimensional variations canbe compensated by laser trimming or by impressing a compensated index ofrefraction grating. Further, the strength of the grating can bedramatically increased by in-diffusing a photosensitizing gas during thegrating writing process. For improved spectral characteristics thegrating is apodized and unchirped by being written with concurrentgrating modulated (a.c.) and uniform (d.c.) intensity UV beams. Size andother characteristics of the waist region are selected such that thedrop wavelength of the coupler is adequately separated from thebackreflection wavelength and the latter wavelength lies outside thefrequency band of interest.

A small coupler having these properties and wavelength adjustability aswell is enclosed within a prepackage structure which enables opticalaccess to the coupler waist for grating writing. An elongated structureconsisting of materials having different thermal coefficients ofexpansion is arranged to compensate the temperature dependence of thedrop wavelength. Moreover, the structure provides fine tuning so thatthe drop wavelength is precisely adjusted and subsequently maintainedthroughout the desired operating temperature range.

Methods and apparatus for writing high strength, precisely definedgratings in very narrow optical fiber structures utilize precisionmechanisms and optical subsystems as well as unique processing steps. Amerged waist region forming a non-evanescent coupler is first formed ina manner rendering the coupler polarization insensitive. Shape controlof a precise nature is achieved by analyzing polarization responseduring elongation as a traversing flame or CO₂ laser beam softens thefibers, and using the measured polarization response to control the heatsource in a manner that minimizes polarization sensitivity. Because ofthe minute diameter of the elongated waist region very small widthvariations can affect grating uniformity, but this is compensated byvarying the background index of refraction. The waist region is renderedphotosensitive to impinging UV light by performing all writingoperations within a pressurized hydrogen or deuterium environment whichassures in-diffusion of photosensitizing gas and replenishes interiorgases as the reaction proceeds, maximizing grating strength anduniformity.

To compensate index of refraction variations along the length of thewaist region, a test grating is written along the waist and thewavelength response is measured at progressive locations along thewaist. In accordance with the readings the background index is variedlocally so that a net equalized value exists along the region in whichthe grating is to be written. During writing the laser beam spot, whichis large relative to the waist, is positioned accurately by use of aservo system which adjusts beam position in response to the lighttransmitted through the fibers. An apodized grating is written in thewaist by dividing a scanning laser beam, in accordance with theapodization function, into a periodically varying (i.e. a.c.) beam, anda d.c. beam. These are concurrently directed onto the waist region tocreate an apodized pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention arises by reference to thefollowing description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a simplified and idealized view of the principal parts, namelythe tapered coupling branches and the waist region, of a coupler inaccordance with the invention, useful in describing the opticalwaveguide modes and characteristics;

FIG. 2 is an enlarged cross sectional view of the asymmetric waistsection of the coupler of FIG. 1, with the extent of the optical waveenergy propagating along the waveguide denoted by dotted lines;

FIG. 3 is a pair of graphs (A) and (B) illustrating the relationshipbetween normalized propagation constants and V number for waveguideconfigurations employed in these devices, useful in understanding howlossy cladding modes are eliminated, how an adequate separation betweendrop wavelength and back-reflection wavelength is achieved, and howdiameter uniformity tolerances relate to coupler diameter;

FIG. 4 is a simplified and idealized view of a coupler twisted at thewaist region to impart polarization independence;

FIG. 5 is a graphical representation of the drop channel spectralcharacteristics; namely, the drop reflectivity versus wavelength,depicting the effect of twist on the polarization splittingcharacteristics of the drop channel;

FIG. 6 is an example of effective modal index of refraction variationsinduced by dimensional variations (top) and the complementary materialor background index of refraction variations along the waist region,which cancel or compensate the variations in mode index caused bydimensional variations. This uv trimmed waist depicts how a uniformgrating is achieved in a small non-uniform diameter waist region;

FIGS. 7(A) and 7(B) are illustrative graphs (not to scale) of UV inducedindex of refraction variations in a coupler corresponding to an apodizedgrating, such as a cosine-squared or Gaussian apodization function;

FIG. 8 is a break-away perspective view of an exemplary coupler inaccordance with the invention having a cylindrical housing and aninterior optical fiber support structure or prepackage;

FIG. 9 is a side sectional view of the coupler of FIG. 8;

FIG. 10 is a fragmentary side sectional view of a fine tuning mechanismfor compensation of wavelength within the coupler of FIGS. 8 and 9;

FIG. 11 is a fragmentary side sectional view of an end portion of thecoupler, showing the manner in which the coupler housing is hermeticallysealed and the exterior fibers are protected;

FIG. 12 is a simplified block perspective and block diagram of a systemfor fabricating a waist region of a coupler while establishingpolarization independence;

FIG. 13 is a simplified representation of an arrangement for writinghigh strength gratings;

FIG. 14 is a block diagram of an instrumentation and scanning system forcompensating for diameter variations in the waist region of a coupler;

FIG. 15 is a schematic representation of an arrangement for writing anapodized grating using a half-wave plate and polarizing beamsplittercombination;

FIG. 16 is a schematic representation of an arrangement for writing anapodized grating using a toggling scanner of varying duty cycle; and

FIG. 17 is a schematic representation of intensity characteristics of(A) a.c. beam and (B) d.c. during beam path toggling with varying dutycycle for a single scanning pass along waveguide.

DETAILED DESCRIPTION OF THE INVENTION

An optical fiber wavelength router in accordance with the invention isexemplified by a wavelength selective filter, here of the type usuallyreferred to as an add/drop filter. Such a device, to which multiplechannels at different wavelengths are applied, redirects in a low loss,highly efficient manner the selected wavelength channel into a firstoptical fiber while transferring the remainder of the channels to asecond optical fiber. While the concepts employed may be used for otherapplications, such as switches, multiple channel routers, andcrossconnects, the add/drop filter is perhaps of greatest immediatebenefit for multiplexers and demultiplexers in wavelength divisionmultiplex (WDM) systems.

FIG. 1 illustrates the physical structure of this device. The fusedcoupler consists of a first fiber 31, 35 and a second fiber 30, 34dissimilar in the vicinity of the coupling region 12 wherein an index ofrefraction grating 27 has been impressed. The two fibers may be madedissimilar by locally pretapering one of them by 20% in the vicinity ofthe fused region. Light launched into the single mode core of upperfiber 31 evolves adiabatically into an LP₁₁ mode with nominalpropagation vector β₁ in the waist, and adiabatically evolves back intothe single mode core of the output fiber 35. Light launched into thesingle mode core of the lower fiber 30 evolves adiabatically into theLP₀₁ mode with propagation vector β₂, and adiabatically evolves backinto the single mode core of the output fiber 34. If an index ofrefraction grating 27 is impressed in the coupler waist 12, and if thewavelength is chosen such that β₁ and β₂ satisfy the Bragg law forreflection from an index grating of period A_(g) at a particularwavelength, say λ_(i):${{{{\beta_{1}\left( \lambda_{i} \right)}} + {{\beta_{2}\left( \lambda_{i} \right)}}} = \frac{2\pi}{\Lambda_{g}}},$

then the optical energy at λ_(i) in the single mode core of the firstfiber 31 is reflected non-evanescently by the grating into the singlemode core of the second fiber 30. The spectral response and efficiencyof this reflective and mode-converting coupling process is dictated bythe non-evanescent coupling strength of the optical modes with thegrating. If the wavelength of the input mode is detuned, say to λ_(j),so that:${{{\beta_{1}\left( \lambda_{j} \right)}} + {{\beta_{2}\left( \lambda_{j} \right)}}} \neq \frac{2\pi}{\Lambda_{g}}$

then the Bragg law is no longer satisfied and the input mode in thefirst fiber 31 travels through the coupler waist 12 and reappears as theoutput mode of the first fiber 35, with minimal leakage into the secondfiber 34. For these wavelengths the coupler is transparent; that is, nocoupling occurs, and the two fused fibers remain optically independent.Therefore, only a particular wavelength λ_(i) is coupled out of thefirst fiber 31, 35 as determined by the grating period in the couplingregion 12.

In addition to backwards coupling of light into the adjacent fiber, thegrating typically reflects some light back into the original fibers atdifferent wavelengths given by 2|β₁(λ₂)|=k_(g) and 2|β₂(λ₃)|=k_(g). Toensure that λ₂ and λ₃ are outside the wavelength operating range ofinterest, the difference between β₁ and β₂ is made sufficiently large.The difference increases as the waveguides become more strongly mergedor as the fused coupler waist decreases in dimension. This general trendis depicted in FIG. 3, whereby the vertical axis separation betweenadjacent characteristic curves for eigenmodes of waveguide generallyincreases for smaller diameters (smaller V's). This difference ismaximized for small coupler waists, where β₁ and β₂ correspondsubstantially to the LP₀₁ and LP₁₁ modes of an air-glass opticalwaveguide. The LP₀₁ mode is a common representation of the HE₁₁ ^(e),HE₁₁ ^(o) modes, and the LP₁₁ mode is a common representation of theHE₂₁ ^(e), HE₂₁ ^(o), EH₀₁ ^(e), and EH₀₁ ^(o) modes, illustrated inFIG. 3. It is common in the art to speak in terms of these LP modes inwaveguide structures such as coupler waists that do not exhibit circularsymmetry.

Furthermore, the tilt angle of the transversely asymmetric grating canbe selected to reduce the coupling strength for backreflection of theLP₀₁ into LP₀₁ modes and the LP₁₁ into LP₁₁ modes. The otherconsideration in selecting angle is to maximize the mode conversionefficiency of the LP₁₀ into LP₁₁ and LP₁₁ into LP₀₁ modes. The typicalangles to minimize backreflection coupling are in the range of 3 to 5degrees and the angle increases as the coupler waist diameter decreases.This angle is slightly different than the angle to maximize the dropefficiency.

To form this fiber optic coupler, two locally dissimilar fibers arefused to a narrow waist typically 1 to 50 microns in diameter, forming awaveguide in the fused region which supports at least two supermodes oreigenmodes of the composite waveguide. The number of supermodessupported by this composite waveguide structure is determined by theindex profile and dimensions of the structure. When this waveguidestructure is significantly reduced in diameter, the waveguidingcharacteristics resemble that of an air-glass waveguide. The modepropagation behavior of this simplified step index waveguide is thenpartially described by a parameter defined as the V number, whichdecreases as the radius a of the waveguide core is decreased, anddepends on the optical wavelength λ_(o) of the mode, the core indexn_(core) and the cladding index n_(clad):$V = {\frac{2\pi \quad a}{\lambda_{o}}{\sqrt{n_{core}^{2} - n_{clad}^{2}}.}}$

For an air-glass waveguide n_(core)=1.45 and n_(clad)=1.0. For anelliptical cross section waveguide, the first or lowest order mode isnominally LP₀₁ and the second mode is nominally LP₁₁. Typically, higherorder modes exist within the coupler waist, as the total number of modessupported by such a waveguide is N≈V²/2, which is 8-9 for a 4 microndiameter waist at 1550 nm. However, the two lowest order modes areprincipally important in the add/drop operation. In general, a lossypeak appears for each higher order mode greater than two. Because thetwo waveguides are sufficiently dissimilar and the tapered transitionregion is sufficiently long, the input optical modes traveling along thesingle mode cores of the original fibers adiabatically evolve into thesupermodes of the coupling region. Upon exiting the coupling region, thesupermodes will evolve adiabatically back into the original opticalmodes as the waveguide splits into the two original fibers. Thus, theoptical energy passes from the input to the output without beingdisturbed. A typical fiber asymmetry of (|β₁|−|β₂|)/(|β₁|+|β₂|)=0.005and a taper angle of 0.01 radians results in less than 1% in undesiredleakage of optical energy from one fiber to the other. To achieve theasymmetry, a pair of identical fibers can be made dissimilar bystretching (adiabatically pretapering) one fiber in a central region.The two fibers are then merged or joined into one waveguide in thecoupling region, yet the two fibers behave as if they were opticallyindependent. A grating is next impressed in the coupling region toredirect light at a particular wavelength from one fiber to another. Forexample, a 125 micron diameter fiber is pretapered by 25%, thenelongated and fused to another 125 micron diameter fiber to form a 4.5micron diameter, 2 cm long waist region with taper lengths of 2 cm. Theresulting taper angle is sufficient to produce a low loss, adiabatictaper. For a UV impressed grating period of 0.540 micron, the wavelengthof the drop channel of representative devices is in the 1550 nm range.

A suitable starting fiber from which such a coupler may be fabricated ischaracterized in part by a photosensitive cladding which may bemanufactured using known fabrication procedures by doping the claddingregion at least partially with a photosensitive species whilemaintaining the waveguiding properties (i.e., the N.A.) of a standardsingle mode core fiber. The goal of the deposition processes for use inthe present invention is to dope a significant volume fraction of thecladding. The farther the dopant (e.g., Ge) extends out along the radiusof the fiber, the more photosensitive the resulting coupler waist willbe after the fusion and elongation stages. It is also important that thefiber be doped in a manner that minimizes thermal stress and materialproperty mismatch within the doped cladding.

WDM systems enable multiple wideband signals to be transmitted on asingle optical fiber, provided that individual wavelengths can beprecisely centered at given values and have narrow bandwidths with highsignal-to-noise ratios. These properties must be provided by theadd/drop filters, and the concept as disclosed and claimed in the abovementioned applications have special advantages in these respects.However, the technical requirements are so critical, as is describedhereafter, that production of units in quantity at low cost without theneed for instrumentation, testing and burning-in at each stage, presentsformidable challenges.

As described in the previous applications and seen in FIG. 1, theadd/drop filter, also referred to as a coupler 10, has a narrow waist 12formed by elongation from optical fiber precursors. The waist 12, whichis in the range of 2-3 cm long, comprises a pair of locally dissimilar,longitudinally merged fibers 14, 15 forming a merged region typicallyless than 10 microns in cross sectional dimension. Specifically in thisexample, the waist region 12 is a hybrid dumbbell-ellipsoid incross-section, here having a major dimension (A) of 10 microns or lessand with a minor (B) dimension that provides a 0.82 ratio between theaxes. The hybrid dumbbell-ellipsoid (FIG. 2) is a shape having physicalcharacteristics resembling a cross between a dumbbell shape and anellipsoid. This shape also has a transverse asymmetry best characterizedas a “peanut” or “pear” shape. The asymmetry is the result of theinitial pretaper. The smaller fiber 15 in the waist 12 is prestretchedbefore elongation and merging so that it is about 20% smaller (in thisexample), although the relative size can vary within a range of 10-30%or more. Where the facing sides of the fibers 14, 15 are fused andmerged they introduce irregularity into the ellipsoid and retain theasymmetry of the original fibers. The waist region 12 is preferablyelongated without twist to prevent the loss of the reference planedefining the centers of the original cores, now only vestigial incharacter. Maintaining this reference plane in the prepackage beforeexposure is essential to producing the proper grating tilt asymmetry.

In preparation of a pair of optical fibers that are to be elongated toform the waist region of a grating-assisted asymmetric coupler inaccordance with the invention, one fiber is prestretched so that it isabout 20% smaller than the other. Then, as shown generally in FIG. 11,the two fibers 100 are engaged at both ends by clamps 101, 102, whichare movable apart at controlled rates by traversing mechanisms 104, 105under control of a programmed computing system 107. Concurrently, atorch 109 heats the glass of the coupler fibers 100, softening them toallow both elongation and fusion along their lengths. Computercontrolled elongation systems of this type are commercially available,and can be employed, with care taken to provide the adiabatic taper ofthe fiber transitions into the waist region and to provide asufficiently uniform diameter waist region.

At each end of the waist 12 the fibers extend outwardly in a divergenttaper 2-3 cm long along separate tapered coupling branches 20, 21 and24, 25. This taper is adiabatic and transitions from the small diameterwaist region 12 to the much larger single mode optical fibers (notshown) which have diameters of the order of 90-125 microns. These fibershave metalized outer surfaces (not shown) suitable for soldering thecoupler to the prepackage and precisely and stably maintaining couplertension after final packaging. Within the waist region 12, a Bragggrating 27 is recorded that is of selected periodicity suitable for thechosen drop wavelength, and the grating planes are tilted (typically3°-5°) with respect to the larger of the transverse dimensions of thewaist 12. A multi-wavelength input propagating along one branch, e.g.,the first tapered coupling branch 21 into the waist region 12 isselectively filtered by the Bragg grating 27, which couples only thedrop wavelength into the second tapered coupling branch 20 and the otherfiber 30.

In accordance with the invention, the modal relationships, dimensionsand properties of the coupler are selected and modified such that anumber of advantageous properties are concurrently achieved. Referringnow to FIG. 2, the reduced diameter waist sections 14, 15, derived fromprecursor fibers are doped to be photosensitive (8 mol % germanium issuitable) in the original cladding region surrounding the small higherindex of refraction core and have only minute vestigial cores afterelongation. Energy is thus confined and propagated in what may be calledan air-glass waveguide, the term “air” here meaning the surroundingenvironment about the physical fiber, whether air or some other medium.Some characteristics of such an air-glass waveguide include a largenumerical aperture and multimode waveguiding properties. The radialextent of the field outside the fiber is represented by the dashed line17.

Within the air-glass waveguiding region of the waist (FIG. 2), theorthogonal optical modes completely occupy and overlap the internalvolume of the adjacent fiber 14 or 15, regardless of whether the lightoriginated in fiber 31 or 30. Because of this complete mode overlap,when a grating is impressed within the waist region, the coupling is“non-evanescent”, although the modes completely overlap with thegrating. Note that the optical mode originally associated with aparticular fiber is not localized within that original fiber region inthe coupler waist. The modes in the waist are no longer waveguiding intheir original fiber material alone.

The air-glass waveguiding property of the coupler waist leads to uniqueoptical characteristics. First, all lossy cladding modes are eliminated.Unlike the precursor optical fiber, whose cladding-air interface alsoserves as a waveguide, the coupler waist has a new uniform cladding(air) that does not support secondary guiding. The waist supportsmultiple optical modes, but their number decreases as the diameterdecreases. However, a very small waist diameter reduces the number ofhigher order modes that degrade the short wavelength transmission ofthis device. These modes are guided in the waist region, yet they escapefrom the fiber in the adiabatic transition regions of the taper sectionsand contribute only to background loss at particular wavelengths. Inaddition, by proper tilt asymmetry of the grating, the coupling strengthto these higher order modes can be dramatically reduced or suppressedentirely.

These characteristics become clear upon analyzing the mode diagrams ofelliptical cylinders representative of coupler waists, depicted in FIG.3. Each curve represents one particular mode supported by the waveguide.FIG. 3 [taken from Lewis, J. E. and G. Deshpande, “Modes on ellipticalcross-section dielectric tube waveguides”, Microwaves, Optics, andAcoustics, Vol. 3, 1979, pp. 147-155] illustrates the normalizedpropagation constants for modes of a coupler waist with an ellipticityof 0.9 (i.e. greater than the present coupler example of 0.82). The topfigure (A) illustrates the odd modes, and the bottom figure (B)illustrates the even modes. The horizontal axis corresponds to the Vnumber of the waveguide, and the vertical axis corresponds toβ/β_(o)=n_(eff), equivalent to the modal index of refraction of theindividual optical modes of the waveguide.

The waveguide characteristics may be expressed in terms of differentmode expressions. For example, the LP₀₁ (linearly polarized) mode isequivalent to a linear combination of the even and odd HE₁₁ modes, andthe LP₁₁ mode is equivalent to a linear combination of the even and oddHE₀₁ and HE₂₁ modes. LP mode descriptions assist in the analysis ofpolarization behavior. The mode evolution properties of ellipticalwaveguides are more amenable to an LP mode description.

The slope of these characteristic curves is a measure of the effectivemode index sensitivity to diameter variations. The greater thesensitivity, the greater the chirping or broadening of the Bragg gratingdue to a given magnitude of diameter non-uniformity. For smallerdiameter couplers (smaller V's) the slope increases and the diametersensitivity increases. That is, smaller diameter couplers have morechallenging diameter uniformity requirements to achieve a narrowspectral bandwidth grating. In addition, the separation betweeneffective index for the LP₀₁ and LP₁₁ modes increases, corresponding toa larger wavelength separation between the drop and backreflectionwavelengths (which can be important, as noted below). The separationbetween these modes and all the additional higher order modes alsoincreases, ensuring that the lossy peaks associated with coupling tohigher order modes are pushed out of the spectral region of interest(e.g., the 1530-1560 erbium doped fiber amplifier (EDFA) window).

Unlike fiber gratings, there are no lossy cladding modes whichcontribute to losses in grating assisted mode couplers, because theactual cladding material of the coupler (typically air) does not have asecondary waveguide structure which supports additional optical modes.Only the doped silica coupler waist supports optical propagation.

The grating assisted mode coupler reflects light at a particularwavelength from one fiber back into the same fiber (the backreflection),and reflects light at a different wavelength from one fiber into theother (the add/drop). The add/drop response leads to the desiredwavelength routing of light from one fiber to another, while thebackreflection response is usually undesirable. Therefore, thewavelength at which the backreflection occurs should lie outside theoperating wavelength region of the add/drop filter. For example, fordense WDM applications in the 1530 to 1565 wavelength range, thebackreflection wavelength should be either below 1530 nm or above 1565nm, or lie at a wavelength between the active wavelength channels. Thebackreflection/drop wavelength splitting should be 18 nm or more.

This wavelength splitting is readily achieved by making the waist of theadd/drop coupler sufficiently narrow (<7 microns) such that thewavelength of the backreflection is far from the drop wavelength. Byfabricating fused couplers with small waists, the difference between themodal propagation constants of the LP₀₁ and LP₁₁ modes increases.Therefore, the wavelength splitting of the drop and backreflection alsoincreases. This wavelength splitting is in excess of 15 nm for anelliptical cross section waist with a major axis of approximately 3.5microns using a specialty doped starting fiber. Further reduction in thecoupler waist diameter leads to a further increase in wavelengthsplitting. The exact relationship between waist diameter and wavelengthsplitting depends strongly on the physical shape and the exact index ofrefraction profile of the coupler waist. A general rule would be to makethe waist smaller than 5 microns. However, the required uniformity ofthe coupler waist diameter becomes increasingly stringent as the waistdiameter decreases; therefore, the waist diameter is usually selected tobe that diameter which gives a backreflection/drop wavelength splittingslightly larger than 15 nm. A given add/drop filter has a backreflectionpeak on either the short (for pretapered fiber input) or long (fornon-pretapered fiber input) wavelength side of the drop peak.

For many telecom applications of add/drops, such asmultiplexers/demultiplexers, this drop/backreflection wavelengthsplitting requirement is substantially relaxed to a splitting on theorder of a WDM channel spacing (0.8 nm or 1.6 nm, for example).Therefore, larger splittings are not be necessary if the wavelengths aredemultiplexed from the fiber in a sequential manner (shorter wavelengthsto longer wavelengths, for example). Even though the longer wavelengthdevices have short wavelength backreflections, those channels at thesewavelengths are already extracted from the fiber by the previousadd/drops. Thus for these units, this waist diameter may be larger,reducing the diameter uniformity tolerance of the coupler.

For most telecom applications the polarization properties of the couplerare important. Optical fields are vectorial in nature; that is, theyhave direction. This direction is quantified by the state ofpolarization of the optical field. The polarization of an optical signalmay be linear, circular, elliptical, or unpolarized. Two linearlypolarized optical signals are othogonally polarized if the electricfield vectors lie perpendicular to one another. For example, the LP₀₁and LP₁₁ modes can be substantially polarized along the x and ydirections, where x and y are the major and minor axes of an ellipse.

The grating assisted mode coupler can readily exhibit a polarizationdependence of the wavelength of light coupled from the input fiber tothe drop. This polarization dependence is due to the form birefringenceof the coupler waist in the region of the Bragg grating. In general, themodal propagation constants β within the waists of fused couplers, forlight in the two orthogonal polarization states, are not equal.Referring to FIG. 4, it can be seen, by referring to the two dotted linepeaks, that a wavelength separation exists between the two orthogonallypolarized modes under these conditions. However, for certain crosssectional shapes and index of refraction profiles of the waist, thepolarization dependence vanishes (i.e.,|βLP_(0.1,x)|+|βLP_(11,x)|=|βLP_(01,y)|+|βLP_(11,y)|). Note that theleft and right sides of this equation are equal, even thoughindividually |βLP_(01,x)| is not equal to |βLP_(01,y)| and |βLP_(11,x)|is not equal to |βLP_(11,y)|. In fact, counter-intuitively, thepolarization dependence of the drop wavelength of a coupler waist ofcircular cross section does not vanish, because the polarizationdegeneracy of the LP₁₁ mode does not vanish for a circular waveguide(that is, |βLP_(11,x)|≠|βLP_(11,y)| for a circular waveguide), while thepolarization dependence of the LP₀₁ modes does vanish(|βLP_(01,x)|=|βLP_(01,y)|).

One waist cross sectional shape for which the polarization splittingdoes vanish at the drop channel is the hybrid dumbbell-ellipsoid with aratio of minor to major axes of about 0.8. Alternate descriptors include“pear” or “peanut” shaped. Such a waist cross section is achieved whenelongating a fused coupler under tension by heating it with a highlycontrolled and repeatable heat source that is varied in temperature andexposure time to achieve the desired cross section until the monitoredpolarization characteristics disappear. Examples of suitable heatsources are well known in the art and include CO₂ lasers, gas flames andresistive heaters. Alternate waist cross sections have also beendesigned to eliminate polarization dependence but the hybriddumbbell-ellipsoid is more readily fabricated. It has been demonstratedthat polarization dependence can be reduced to <<0.1 nm by manufacturingthe coupler so that its waist has a precise amount of shape asymmetry,as with the preferred elliptical shape. The present add/drop filter hasbeen fabricated in a manner that ensures that the polarization splittingof the add/drop wavelength is less than 0.05 nm. The operation of such adevice is then essentially polarization insensitive for gratings of FWHMbandwidth a few times the polarization splitting, or about 0.2 nm. Theoptical transmission spectra are then independent of the polarization ofthe input signal. Under such conditions the orthogonally polarized modesmerge into the single drop wavelength, as shown in FIG. 5.

The coupler workstation (FIG. 12) enables control of the coupler waistcross sectional shape. In this instance, however, the torch 109 isdriven in bi-directional fashion by a reciprocating drive 111 undercontrol of the computer 107, and also fed an adjustable mix of gasesthrough computer controlled mass flow controllers 114. In this example,the gases are H₂, O₂ and N₂. By varying the N₂ flow while keeping the H₂and O₂ flow the same, the regulator 114 can regulate the flametemperature without substantially changing the flame geometricalcharacteristics.

The objective is not only to draw down to a waist region as the fibers100 are elongated, but at the same time to shape the fiber cross-sectionso as to minimize polarization dependence. For this purpose a lightsource 121 attached to the ends of the fibers 100 sends a light signalthrough the fibers 100 to a polarization analyzer 123, such as a HewlettPackard Model 8509B. This instrument monitors the rotation of thepolarization in the LP₀₁ and LP₁₁ modes transmitted through the coupleras it is fused and elongated. This allows the birefringence of thecoupler waist to be measured for the two lowest order modes coupled bythe grating. The coupler is then fabricated by adjusting heat exposureduring elongation so that the birefringence as measured by thepolarization analyzer is equal and opposite for the LP₀₁ and LP₁₁ modes,the condition for polarization independence.

Careful manufacturing process control may be required to repeatedly getthe shape of the waist correct. Therefore, an alternative techniquewhich can null polarization dependence of the add/drop device after thecoupler has been manufactured can be of practical importance whetherused in addition to or separately from the shape control. The grating isfirst recorded in a coupler, and twist is then applied to the couplerwaist until the polarization splitting of the central peak vanishes, asshown in FIG. 4. The coupler is then packaged in this twisted state topreserve the polarization independence. As another alternative, forsuitable UV grating writing conditions (e.g., polarization andintensity), polarization dependence can be trimmed out by the UVexposure. The UV exposure process produces material birefringence withinthe glass that can compensate for the form birefringence of the couplerwaist.

One technique to reduce polarization splitting of the coupler utilizestwisting of the coupler waist. Conceptually, the polarization eigenmodesof the coupler waist are linearly polarized. As the coupler is twisted,the polarization eigenmodes become elliptically polarized. In the limitof very large twist they become circularly polarized. The propagationconstants for the two orthogonal polarization modes in a twisted couplerwaist are given by:

β±=(β_(x)−β_(y))/2±ξ{square root over (1+((β_(x)−β_(y))/2ξ)))}

Here ξ is the twist of the coupler in rad/m, and β_(x), β_(y) are thepropagation constants of the untwisted coupler waist. The wavelengths ofthe three peaks in the drop spectrum are determined by the grating phasematching conditions, where k_(g) is the grating vector:

β₊(λ₁)+β(λ₁)=β(λ₁)+β₊(λ₁)=k _(g)

β₊(λ₂)+β₊(λ₂)=k _(g)

β(λ₃)+β(λ₃)=k _(g)

Input light polarized along y transforms into the − mode, and isreflected by the grating into a superposition of backwards-propagating +and − modes through the first and third phase matching conditions,respectively, giving rise to two peaks at wavelengths λ₁ and λ₃. Inputlight polarized along x similarly is reflected through the first andsecond phase-matching conditions at wavelengths λ₁ and λ₂. The firstphase matching condition, which is common to both input polarizations,gives rise to the central unpolarized peak at λ₁. The second and thirdphase matching conditions give rise to the polarized sidebands at λ₂ andλ₃.

The generic technique of applying twist to optical waveguides has beenused by several researchers to control birefringence, as by Birks (tonull the polarization splitting of an acousto-optical switch). Wilkinson(to control the birefringence of a wavelength-division multiplexer madefrom a biconical taper coupler) and Barlow et al. (to null thebirefringence of a singlemode optical fiber). Using twist in this mannerto control the birefringence for light that is forward-coupled or simplypropagates through the coupler has different requirements thancontrolling the polarization splitting for backward-coupled light.

A grating can be imprinted by a UV side-writing technique, although themethod for producing the grating is unimportant. The coupler is thentwisted about the axis of the coupler waist. As this is done, the twodrop peaks associated with the two orthogonal polarizations of lightwithin the coupler will be joined by a third peak at their averagewavelength. This third peak will show no polarization dependence. As thenumber of twists is increased, the two polarization peaks will increasetheir separation and diminish in strength while retaining theirpolarization dependence, and the central peak will grow to twice thestrength of the original polarization peaks. This is depicted generallyin FIG. 4.

Arbitrarily large twist will produce arbitrarily large separation andarbitrarily weak strength of the polarization peaks. The maximum amountof twist is determined by the mechanical properties of the glass fiber.The appearance of the central, polarization independent drop peak andtwo polarization dependent sidebands upon twisting is unique to thisreflection-mode add/drop device. In practice, the coupler is twisteduntil the sidebands are sufficiently weak, after which the coupler ispermanently packaged with the given amount of twist.

The response of a reflection grating in a coupler waist is oftenundesirably “chirped” or spectrally distorted if the diameter of thewaist is non-uniform, because the local propagation constants and dropwavelengths vary with changing diameter. Therefore, a grating ofconstant period within a non-uniform waist will have a broader spectralwidth than a grating of constant period within a uniform waist. Agrating with a 1 Angstrom spectral width requires that the variation indiameter be less than 0.01 microns for a 5 micron cross sectionalcoupler waist over that region of the waist containing the grating.Similarly, the shape of the coupler should be constant over the regioncontaining the grating to prevent additional chirp and polarizationdependence. The grating chirp in the grating response due to smallvariations in the diameter of the coupler waist is substantially reducedor effectively eliminated, in accordance with the invention, by UVtrimming the background d.c. index of refraction within the gratingregion to compensate in the local modal propagation constants along thewaist.

This confronts one of the key issues in the manufacture of acommercially viable add/drop filter based on grating assisted modecouplers. The first is to develop a high yield exposure process toproduce narrow bandwidth gratings, such as those required for 100 and 50GHz WDM systems. A technical hurdle to realize such a device is couplerwaist uniformity. Fused coupler waists inherently exhibit slightdiameter non-uniformities (typically 100 nm in size) that broaden thegratings by approximately a nanometer for coupler waists 4.5 microns indiameter. To produce 0.1 nm bandwidth gratings requires thatnon-uniformities lie below 10 nm.

In practice it is difficult to reduce the coupler diameternon-uniformities below 50 nm. However, arbitrarily narrow gratings maybe produced within non-uniform coupler waists by locally changing thematerial index of refraction by locally UV trimming or exposing thoseregions which are smaller in diameter than the largest diameter segmentwithin the region to be exposed. This relationship is depicted generallyin FIG. 6. The local modal index of refraction as a function of thedistance z along the coupler is a function of the material index of theglass and the waist diameter. To compensate for variations in the waistdiameter, the d.c. or background value of the index of refraction islocally tailored by a scanning UV beam to cancel the variation in themodal index. This provides a means for post-processing after fiberelongation to reduce any broadening of the gratings due to diameternon-uniformities.

To determine the non-uniformities of diameter in a manner that can bescaled up to a manufacturable process, they can be directly measured byscanning electron microscopy, atomic force microscopy or by opticalmeans. However, it has been found that a more precise measurement can bebased on measuring the local Bragg wavelength of a uniform period butweak test Bragg grating recorded within the non-uniform waist.

In addition to producing uniform gratings within fused couplers,precisely apodized gratings are necessary to reduce grating sidelobesand eliminate adjacent channel crosstalk. Apodized gratings are key tomeeting the performance requirements of WDM systems. Apodization isunderstood to have been achieved by several methods, including variablespeed scanning, dithering of the phase mask and apodized phase masks.

An apodized grating can be written by spatially varying the modulationamplitude or a.c. component of the index of refraction in thelongitudinal direction along the grating. At the same time, however, thed.c. or background index of refraction must remain extremely uniform(variations less than 0.0001) to prevent undesirable chirp or broadeningof the grating. Raised cosine (cos²(z)), sinc²(z), and Gaussian(exp−z²/2σ²) apodization functions are both effective in reducing thegrating sidelobes to below −30 dB. However, the raised cosine is a moreefficient apodization shape in terms of reducing the grating exposurelength requirements for a given spectral bandwidth.

An apodized grating exhibits a longitudinally varying index ofrefraction modulation amplitude as well as a uniform period patternalong the waveguide. That is, the grating is gradually (over a largenumber of grating periods >1000) turned on and then off along the lightpropagation direction. This smoothly varying window function reduces thespectral ringing or sidebands resulting from gratings with a sharp,rectangle window function. In general, the frequency spectrum of thefilter is the Fourier transform of the spatial window function of thefilter. A superior method to achieve apodization is to use a scanningexposure, in which the contrast of the optical interference pattern isvaried as the grating is recorded while the total incident intensity iscontrast. To achieve this, the waist region is simultaneously exposedwith a d.c. beam counter-propagating with the modulated a.c. beam whilethe interference pattern is imprinted. The sum of the intensities of theinterference pattern and the uniform beam are kept constant, eliminatingundesirable chirp arising from variations in the background index ofrefraction. As seen in FIG. 7, the index of refraction function for thecos² apodized grating is thus an apodized periodic wave varying inbipolar fashion about a center line over a grating length L and is givenby:${\Delta \quad {n(z)}} = {\Delta \quad {{n_{o}(z)}\left\lbrack {1 + {\sin \quad k_{g}z\quad \cos^{2}\frac{\pi \quad z}{L}}} \right\rbrack}}$

The intensity of the a.c. beam is:

I(z)=I _(o)(sin k _(g) z+1) cos² πz/L

and the intensity of the d.c. beam is:

I(z)=I _(o) sin² πz/L.

Important advantages of the invention also reside in the featuresincluded in the example of FIGS. 7-10, to which reference is now made.The add/drop coupler device 50 comprises a cylindrical housing 52 ofstainless steel tubing that has a 0.270″ OD and a length of 3.67″ andwhat may be termed a “prepackage” or support structure 54 internal tothe housing 52. The prepackage structure 54 is inside the housing 52after assembly but used as a preliminary retainer to hold in the opticalfiber coupler 53 precisely during processing steps in which the gratingis written and adjustments are made. The prepackage structure 54 extendslongitudinally along and within the housing 52, and centrally supportsand retains the optical fiber coupler 53, in position along theapproximate central axis.

In an initial assembly the opposite ends of the optical fiber coupler 50are fixed to spaced apart brass end hubs 58, 59 on a pair of parallelinvar rods 62,63 that extend along the majority of the inside length ofthe housing 52. For solderability and freedom from contamination, thehubs 58, 59 and rods 62, 63 are nickel plated, preferably by anelectroless process, as are the other elements within the housing 52.

The prepackage structure is completed by interposition between the endhubs 58, 59 of a pair of spaced apart base hubs 66, 67, one of which isproximate to the end hub 59 and is soldered or welded to the first invarrod 62. The other base hub 66, on the second rod 63, is adjacent areference hub 69 on the first end hub 58 side, also on the second rod63.

In the prepackage assembly and adjustment phase, there are twosubassemblies of rods and hubs, longitudinally slideable relative toeach other. One subassembly comprises the first invar rod 62, the firstend hub 58 and the second base hub 67, each hub being fixed in positionon the rod 62. The other assembly comprises the second invar rod 63, thereference hub 69, the first base hub 66 and the second end hub 59.Engaged in this way, the whole assembly may be mounted in a fitted jigor tray (not shown) with the waist region of the optical coupler 53being open, for writing of a Bragg grating, to an optical system mountedon the side.

The optical fibers in the coupler 53 diverge from the waist region ateach end to where they are fusion spliced to metallized optical fibersof standard dimension. The entire optical coupler 53 extends along theapproximate central axis of the housing 52, passing through radial slots70 provided in each of the hubs 58, 59, 66, 67 and 69. When theprepackage structure 54 is held rigidly in place in its positioningtray, the coupler 53 can then be soldered at its end regions to thecentral regions of the spaced apart end hubs 58, 59. The waist region isthus stably configured for photosensitization and grating writing steps.

When a grating is written in the waist region with a selectedperiodicity its drop wavelength must be adjusted to sub-nanometerprecision and this wavelength should be essentially constant over therequired operating temperature range, normally −35° C. to 85° C.Although invar has a very low temperature coefficient, it alone cannotmeet the athermal requirement. The prepackage requirements are that theseparation between the end hubs 58, 59 decrease as temperature isincreased. This decreases the tension and resulting strain ε within thecoupler waist with increasing temperature T is a manner that satisfiesthe following equation:$\frac{\partial ɛ}{\partial T} = {{{- \frac{\Lambda_{g}}{n_{eff}}}\frac{\partial n_{eff}}{\partial T}} - \frac{\partial\Lambda_{g}}{\partial T}}$

For Ge-doped silica glasses, the primary contribution to the temperaturedependence arises from the first term on the right of the aboveequation; that is, the temperature dependence of the effective index ofrefraction n_(eff). The second term on the right, the thermal expansioncontribution to the change in grating period, is typically an order ofmagnitude smaller than the first term.

Note that the base hubs 66, 67 include aligned longitudinal grooves 72,73 respectively, in their peripheries, and that an adjustment screw 75extends through the reference hub 69 and the first base hub 66.Consequently, after first adjusting the end hubs 58, 59 to tune thegrating, the wavelength is locked in by soldering a stainless steel rodto grooves 72, 73 in the base hubs 66, 67. This inner structure withinthe prepackage establishes an interior length which has a differentthermal coefficient of expansion than the invar rods 62, 63 which areseated at each end but not otherwise spatially defined except throughthe interior stainless steel connection. Each base hub 66, 67 is coupledto a different invar rod 62 or 63 respectively but has a differentspacing along that rod from the end hub 58 or 59 which determines thegrating periodicity.

With the stainless steel rod 80 in place, the unit can be inserted intoa temperature controlled chamber and cycled through the requiredtemperature range while reading the drop wavelength with opticalspectrum analyzer instrumentation. To adjust the drop wavelength so thatit is the same at 25° C. as 85° C., the adjustment screw 75 is threadedinwardly or outwardly relative to the reference hub 69. As best seen inthe fragmentary view of FIG. 14, the screw has a first short thread 76mating in the reference hub 69, and a terminal second thread 77 matingin the first base hub 66. By turning the screw 75 at the screw head 78,the screw engagement point with reference hub 69 can be shiftedlongitudinally, increasing or decreasing the length of one invar segmentand increasing or decreasing the length of the stainless steel segment.

When the prepackage structure 54 including the optical coupler 53 isadjusted, it is removed from the holder or tray and inserted into thecylindrical housing 52. The prepackage 54 is fixed in position relativeto the housing 52 simply by crimping the housing 52 (see FIG. 7), ontothe second end hub 59. End caps 82, 83 with central bores 83 providingopenings for the fibers are engaged into the housing 52 open ends, andsoldered or welded into place. The outwardly extending fibers at theexit points are soldered to the end cages 82, 83 to produce hermeticseals. Preferably, for longer life, the housing 52 is filled with aninert gas before the housing 52 is sealed. The outwardly extendingfibers are protected against kinking and strain by shrink fit tubes 87,88 of a suitable length.

To reduce the propagation of cracks within the coupler waist and failureof the coupler after packaging, the coupler should be hermeticallypackaged. The presence of water within the package can lead to couplerfailure. This problem is exacerbated when the coupler is packaged undertension, which is necessary to provide a temperature insensitive mount,for example. Preferably, the coupler is packaged either in argon,helium, nitrogen gas or a mixture thereof, or in vacuum.

Obtaining a strong (>99.9% efficiency) Bragg grating in a fiber ismaterially dependent on the photorefractive properties of the fiber.With a small diameter waist region in the fiber body, particularly onein which a more highly doped core has been stretched to a non-activediameter, the grating is written in what previously would be termed thecladding of the precursor optical fibers. The claddings of typicalsingle mode fiber used to fabricate fused couplers are not doped in amanner providing cladding photosensitivity.

Typical methods disclosed in the prior art to post-photosensitizesuitably doped optical fibers to UV illumination are further ineffectivefor relatively small coupler structures. The waist of a coupler of thepresent invention is typically only 3-5 microns in diameter and severalcentimeters long. The waist lies approximately at the center of thefused region of the two starting fibers. If the as-fabricated coupler issubsequently UV exposed in the waist, the drop efficiency is typicallyless than 50%. To increase the grating strength above 50%, variousmethods of post-processing the physical coupler structure can beapplied; for example, flame brushing or hydrogen loading. However, flamebrushing of small, fragile coupler waists easily distorts and bows thefiber under the force of the flame. The abrupt and non-adiabatic bendingcan lead to loss and failure of the coupler. A superior method to UVphotosensitize a coupler waist, referring now to FIG. 13, exposes thecoupler waist in a pressurized vessel 130 containing hydrogen and/ordeuterium gas. The gas pressure is typically 1000-5000 psi, although ifthe coupler or gas is heated, for example to 150-300° C., the hydrogengas will diffuse into the glass very rapidly (<sec) and lower pressuresmay be equally effective at photosensitization. The fused fiber lengths130, supported at opposite ends in a holder 134, such as the prepackagestructure previously mentioned are illuminated by a laser 136 beamthrough a window 138 of the chamber 130. The laser beam is directed by ascanning optical system 140 to traverse along the coupler length througha mask 142 which provides the needed periodicity in the written pattern.This is only a generalized depiction, but the principle is applicablewhatever type of optical pattern is being implemented.

FIG. 13 also shows, however, an advantageous tracking system used toscan a tightly focused laser beam uniformly along the coupler waist.Focusing and shaping optics form a beam which is of about 20-200 umtransverse to the fiber and 200-500 um longitudinal to the fiber.Repeatable positioning of the waist region with the precision that wouldbe needed to overlap with this narrow spot size is not feasible, andadditionally the fiber can shift during exposure. Accordingly, an activepositioning system implementing a tracking algorithm in the hostcomputer 144 to control a galvanometer driven mirror is used in thescanning system 140. The computer 144 receives the signal from aphotodetector 146 responding to 400 nm fluorescence that is generated inthe coupler waist by the impinging beam from the laser 136 (here a 100mW cw frequency doubled argon laser at 244 nm). The tracking algorithmmaximizes fluorescence and hence optimizes the laser beam alignmentduring scanning, so that alignment and movement problems are overcome.

In practice, it is challenging to produce a uniform exposure across thewaveguide because of the non-linear nature of the grating exposureprocess and the large UV induced index changes. The induced index changeis a highly sensitive function of the incident intensity and rapidlydiverges for small non-uniformities in intensity during the scanningexposure. Intensity variations may be exaggerated further because of thewaveguide heating variations that result. These non-uniformities mayarise from slight imperfections in the optical transmission of thescanning optics and phase mask, for example. To maintain uniformity ofthe exposure, a feedback system has been developed which maintains aconstant or pre-determined luminescence power for each spatial locationalong the scan. If the luminescence signal deviates from the desiredsignal during an individual scan, the feedback system computes the localcorrections to the UV incident power necessary to maintain the desiredluminescence signal for the next scan pass. An algorithm is developedwhich maximizes the UV incident power and maintains the desiredluminescence signal for each pass. In the case of negligible 400 nmabsorption, the desired luminescence signal is a constant value for eachpass. The UV power on each pass is modulated by a halfwave plate andpolarizing beamsplitter combination, for example, synchronized with thelongitudinal scanning motion of the exposing beam along the waveguide.Other well known methods to modulate laser power can also beimplemented. This feedback process continues for each scanning pass, andthe compensating UV power function is continually updated following eachpass. This system has successfully maintained the exposure constantacross the grating region to within better than Δn=0.0005 for aΔn_(total)=0.01 UV induced index of refraction change. A system toachieve this exposure uniformity is a combination of the scanningexposure system of FIG. 13 and the modulation scheme of FIG. 15, bothcontrolled by a host computer 144.

The waist is exposed in the hydrogen/deuterium-based atmosphere at 1500psi during the exposure and continually in-diffusing hydrogen/deuteriumreacts to form O-H or O-D within the optical fiber as the exposurecontinues. The hydrogen/deuterium is continually replenished within thecoupler waist by continued in-diffusion of gas from the surroundingpressure vessel until the exposure is complete. The rate of diffusion isenhanced by the UV induced heating of the glass; otherwise, an externalheat source such as a CO₂ laser can be applied. The optimal temperature,dictated by the hydrogen in-diffusion time, the rate of O-H formationduring exposure and grating annealing considerations, is in the range of100-300° C. Thus, the concentration of hydrogen/deuterium gas within thefiber remains at a relatively constant value during the entire exposure,while the amplitude of the periodic modulation of the O-H or O-Dconcentration continues to increase as the coupler is exposed. UVinduced index changes as large as 0.010 can be readily achieved. Theshort diffusion times unique to the fused coupler waist structure arenot counteracted by equally short out-diffusion times to a non-hydrogenatmosphere.

This photosensitization technique has several advantages over earlierfiber grating hydrogen loading techniques. Prior techniques did notexpose the fiber while maintaining it within a hydrogen environment;therefore, if these earlier techniques were applied to fused couplerwaists, the hydrogen gas would leak out too rapidly before and duringexposure to be effective. Typical exposures take in excess of 15 minutesto complete. Second, the simultaneous hydrogen treatment/exposurerequires a significantly lower hydrogen pressure than the usual loadingtechniques. Typically, in the prior art the glass has been loaded to alevel such that the hydrogen concentration is of the order of theGermanium dopant concentration in the glass. This requires hydrogen gaspartial pressures of 20,000 psi for the high dopant levels typicallyused. However, by exposing the coupler within a hydrogen/deuterium gasenvironment, equally large effective O-H or O-D concentrations withinthe glass at the end of the exposure are achieved even if the gaspressure is an order of magnitude lower.

In the prior art, the glass waveguide is pretreated with hydrogen ordeuterium gas, and is removed from the pressure vessel and thensubsequently exposed outside the pressure vessel. This pretreatmentprocess is not optimal for treating fused coupler waists. Once thecoupler is loaded and removed from the high pressure hydrogen chamber atroom temperature, gas rapidly escapes from the 3-5 micron diameterwaist. In less than 30 minutes, more than 95% of the hydrogen hasescaped from the center of the waist. To compound this problem, thewaist may heat up significantly (>100° C.) during the subsequent UVexposure. At these temperatures, hydrogen gas out-diffuses from thewaist in a matter of seconds. Since grating recording typically takes atleast tens of seconds, most of the hydrogen escapes before the exposureis complete. Therefore, this hydrogen loading technique is not preferredfor recording strong UV index of refraction gratings in narrow couplerwaists. However, the technique has been improved by cooling the couplerwaist, both prior to and during the exposure. A temperature in the rangeof <0° C. significantly limits out-gassing during the exposure stage.

As discussed above, a very narrow waist region can have minor widthvariations which profoundly affect the spectral properties of the indexof refraction grating. Spectral properties such as the drop wavelength,filter bandwidth, sideband suppression and crosstalk level greatlyimpact the systems applications of these filters. These width variationsin couplers are effectively neutralized by a compensating approach,instrumentation for which is shown in FIG. 14. The technique ofmeasuring the local periodicity utilizes optical coherence domainreflectometry (OCDR). It is related to a measurement system utilized tomeasure temperature and strain profiles for fiber Bragg grating sensors.

The experimental setup to achieve such a measurement is shown in FIG.14. The light source to interrogate the add/drop filter is a broadbandelectroluminescent diode (ELED) 150 of 30 nm spectral width about 1550nm amplified by an erbium doped fiber amplifier (EDFA) 152. Thisbroadband light is next spectrally filtered by a tunable bandpass filter154 of 3 nm spectral width. This light is input into a optical coherencedomain reflectometer 156. The reflectometer consists of a Michelsoninterferometer with the add/drop filter 160 under test contained in onearm, and the reference arm being of variable optical path length to pathlength match to different longitudinal regions along the coupler waistin the test port arm. The test port arm includes a three port fiberopticcirculator to launch the drop channel back into the reflectometer. Byproperly de-tuning the bandpass filter to lie at the short wavelengthside of the add/drop filter under test, a measure is taken of thereflectivity vs distance along the grating. Since the filtered andamplified light source 150 is of limited spectral bandwidth, thereflectivity signal on the reflectometer 150 is non-zero only for thosegrating regions that have a Bragg wavelength coinciding with thereference filter. If the reference filter is just overlapping the shortwavelength side of the grating in the filter 160, those grating regionsnot exhibiting a reflection require an increase in the local effectiveindex of refraction. This measurement is used to position a laser 162beam (Coherent Innova FRED laser) using a scanning and precisiontranslation stage 140 (see FIG. 12) controlled by computer 144 so thatthe index can be increased by UV radiation in these regions. Thistrimming is performed in the high pressure hydrogen or deuteriumchamber, so significant increases in the index of refraction (0.01) canbe produced with a 1 to 100 Joule exposure. Once the reflection dipsmeasured on the reflectometer 156 are eliminated, the background indexof refraction has been properly trimmed out to eliminate the spectralbroadening arising from diameter non-uniformities. An optical spectrumanalyzer 164 simultaneously monitors the grating response to confirmthat the non-uniformities are being trimmed out of the grating.

In a particular example, the first stage of the compensation exposureconsists of recording a weak, uniform test grating (−10 to −30 dB instrength) in the coupler under fabrication. The grating is nextmonitored at the test port of an HP8504B precision reflectometer whoseinternal 1550 nm ELED source is spectrally filtered by a Micron OpticsInc. 3 nm FWHM tunable Fabry-Perot filter and amplified by an OptigainEDFA Power Amplifier and inputted back into the precision reflectometerexternal light source input. The spectral features of the light inputinto the device under test is simultaneously monitored on an HP71450Boptical spectrum amplifier to determine the local Bragg wavelength ofthe grating. The variations in local Bragg wavelength arise because ofcoupler waist non-uniformities. A computer analyzes the reflectometerdata and filter wavelength to determine the diameter non-uniformitiesand automatically trims the local d.c. index of refraction along thecoupler waist region containing the grating. This trimming enablesnarrow band, low crosstalk add/drop filters to be produced in imperfectfused couplers.

In an alternate example of UV trimming, a map of the local dropwavelength as a function of position along the coupler can be producedby processing the reflectometer and filter spectral data. From thisdata, the coupler can be exposed point-by-point using a scanningexposure system, for example, to correct for diameter non-uniformitiesalong the entire longitudinal extent of the grating. The goal of thecorrection procedure is to prepare a region of the coupler so that thelocal drop wavelength is substantially constant along the region.Alternately, a substantially uniform diameter region may be identifiedthat does not require trimming followed by recording of a stronggrating. Either trimming approach works best for weak test gratings(<10%). Strong gratings exhibit “pump depletion” and multiplereflections within the grating region, both of which degrade theaccuracy of the measurement system. These strong gratings arise fromfirst order diffraction from the index of refraction grating.Alternately, the weaker second order diffraction arising from the Δn²contribution to the well known coupled mode equations for diffractionoff a thick grating can be monitored. Since the second-order diffractionis inherently weaker, this signal can be used to provide an accuratemeasurement of uniformity even for gratings which exhibit a strongfirst-order diffraction.

Once the non-uniformities of the coupler waist are corrected bytrimming, a strong cos² apodized grating is recorded by simultaneouslyexposing the coupler waist with a uniform phase mask from one direction(a.c. exposure) and with a uniform beam from the other direction (d.c.exposure) which does not pass through the phase mask. An optical systemutilizing a computer controlled, rotating half-wave plate in addition toa polarizing beamsplitter varies the ratio of power between the ac anddc beams as the illumination spot is scanned across the fiber. However,the total power incident on the fiber is independent of this ratio andis constant, ensuring that the process of apodization does not inducegrating chirp. The final add/drop has a channel rejection of >40 dB, abandwidth of several Angstroms, an apodized wavelength spectrum, andexhibits a background loss of less than 0.2 dB. A workstation to producesuch apodized gratings is illustrated in FIG. 15. The polarization ofthe exposure beam from a laser 170 is varied by a rotating half-waveplate 172 that rotates a linearly polarized beam by two times the angleθ between the incident polarization and the fast axis of the half-waveplate 172. A polarizing beam splitter (PBS) 174 then splits the incidentbeam into two orthogonally polarized beams traveling along differentoptical paths. The a.c. beam illuminates a phase mask 176 to produce themodulated grating exposure, and the uniform d.c. beam intersects thefiber directly. The ratio of power in the a.c. and d.c. beams is variedaccording to I_(ac)/I_(dc)=cos²(2θ), while the total power I_(ac)+I_(dc)remains constant. By linearly varying the angle θ from 0 to π/4 as thegrating exposure is scanned along the length of the fiber, a cos²apodization is automatically produced. Note that this technique may beapplied to both coupler gratings and fiber gratings.

An alternate apodization approach uses a scanner to toggle a constantintensity beam rapidly between the a.c. and d.c. arms. Such a system isillustrated in FIG. 16.

The input beam from the laser is directed through shaping optics 212into a controllable deflector or toggling mechanism 202 which alternatesthe beam so that it angles off different faces of a prism 204, to followan AC path and a DC path, respectively. In each path corner deflectorsand lenses are positioned to direct the beam from opposite directionsonto the coupler 210, which is held on a fixed optical platform 208. Agrating configured as a phase mask is juxtaposed in the AC path,adjacent the coupler 210 and supported on a movable platform 206 whichreciprocates along the length of the coupler 210. The toggling actionthus alternates the direction of delivery of light energy on the coupler210, but the AC component is distributed by the phase mask, depending onits relative position, while the DC component is unaffected. The dutycycle of this toggling (FIG. 17) is varied with longitudinal distancealong the grating to control the apodization profile. Toggling of aconstant intensity beam (sum of (A) and (B) in FIG. 17) keeps the totalintensity constant on the fiber during exposure. This is essentialbecause the local heating of the coupler is dependent on intensity. Itis necessary to maintain the temperature locally as the beam is scannedto provide a uniformly exposed and annealed grating region.

Although a number of alternatives and expedients have been shown andmentioned, it will be appreciated that the invention is not limitedthereto but encompasses all forms and variations within the scope of theappended claims.

We claim:
 1. The method of providing a wavelength selective opticalcoupler having a narrow bandwidth drop wavelength and high gratingstrength comprising the steps of: elongating a portion of one opticalfiber pair each having a core and cladding to a narrow waist region inwhich the fibers are longitudinally merged and the core is vestigial;tapering optical fiber transitions to and from the waist regionadiabatically to a dimension such that the optical modes of originalfibers evolve adiabatically into orthogonal modes within the waistregion; modifying the polarization dependence of the propagationconstants of the orthogonal modes to establish substantial polarizationindependence of the drop wavelength in the operation of the coupler;photosensitizing the waist region; writing an apodized reflectiongrating having a periodicity adapted for the drop wavelength in thewaist region while maintaining the photosensitization.
 2. The method asset forth in claim 1 above, wherein coupling in the waist region isnon-evanescent and further including the step of compensating forcross-sectional dimensional variations along the waist region by writinglocal variations in the material index of refraction of thephotosensitized waist region.
 3. The method as set forth in claim 1above, wherein the dimensions of the waist region are sufficiently smallto support a limited number of waveguiding modes but not lossy claddingmodes.
 4. The method as set forth in claim 1 above, wherein the waistregion is elongated to be sufficiently small to separate the wavelengthsof backreflections from the grating from the drop wavelength and toplace the wavelength of backreflections outside the spectral band ofinterest.
 5. The method as set forth in claim 1 above, wherein the stepof writing the apodized grating comprises writing both an a.c. componentand a d.c. component, such that the index of refraction variation issymmetrical about a base line.
 6. The method as set forth in claim 1above, wherein the coupler operates in a given wavelength band and theelongation step includes reducing the cross-sectional dimensions of thewaist region to a size for which the backreflections from the gratingare shifted outside the wavelength band.
 7. The method as set forth inclaim 1 above, wherein the optical fibers have cladding dopant renderingthe coupler waist capable of being photosensitized.
 8. The method ofmaking a polarization insensitive optical wayeguide coupler comprisingthe steps of: elongating a pair of optical fibers along a predeterminedcoupler waist length to form non-circular waists which support an LP₀₁mode and an LP₁₁ mode each consisting of two polarizations andpropagating along the coupler waist, and adjusting the formbirefringence of the coupler waist until the LP₀₁ mode birefringence isequal and opposite to the LP₁₁ birefringence.
 9. A method as set forthin claim 8 above, wherein the birefringences are adjusted by analyzingthe rotation of polarization in the two modes of propagation duringelongation and shaping the cross sections of the fibers to achieve anequal and opposite angle of polarization rotation for the LP₀₁ and LP₁₁modes.
 10. A method as set forth in claim 9 above, wherein the twofibers are fused together during elongation and further including thestep of shaping the cross sections by heating the waist with a movingflame of variable temperature during elongation.
 11. A method as setforth in claim 10 above, wherein the step of shaping the cross sectioncomprises propagating light through the waist during the elongation,measuring the rotation of polarization in the light after passagethrough the waist, and varying the flame to control the shape of thewaist.
 12. A method as set forth in claim 11 above, wherein the flame isvaried by controlling the flow rate of N₂, H₂ and O₂ gas to control thetemperature of the flame.
 13. A method as set forth in claim 11 above,wherein the fibers are shaped to a hybrid dumbbell-ellipticalcross-section.
 14. A method as set forth in claim 13 above, wherein thecross section has a minor to major axis ratio of about 0.8.
 15. A methodas set forth in claim 10 above, wherein the flame controls the shape ofthe waist by heating to a temperature sufficient to reduce viscosity ofmaterial in the waist such that a dumbbell cross section of twooriginally fused fibers evolves into a hybrid dumbbell-ellipsoid crosssection and wherein heating is reduced upon attaining the desired crosssectional shape to preserve desired polarization properties.
 16. Themethod of making a polarization insensitive optical waveguide couplercomprising the steps of: elongating a pair of optical fibers toestablish a predetermined merged coupler waist length withcross-sectional configurations which support an LP₀₁ mode and an LP₁₁mode each consisting of two polarizations and propagating along thecoupler waist, and adjusting the uv induced material birefringence ofthe coupler waist until the LP₀₁ mode birefringence is equal andopposite the LP₁₁ mode birefringence.
 17. A method as set forth in claim16 above wherein the waveguide coupler has at least one drop wavelengthand the fibers are fused together during elongation and wherein thebirefringences are adjusted by twisting the waist along its length afterfabrication until the drop wavelengths of the LP01 and LP11 modes aresubstantially polarization independent.
 18. A method as set forth inclaim 16 above, wherein the waveguide coupler has at least one dropwavelength and the modal propagation constants are equalized such thatthe polarization dependence of the drop wavelength is less than 0.05 nm.19. The method of minimizing polarization dependence in a wavelengthselective optical waveguide device having a drop channel and propagatingorthogonal modes such that the drop channel redirects two orthogonalinput polarizations at first and second spaced apart wavelengths,comprising the step of reducing the wavelength differential of the firstand second spaced apart wavelengths to reduce the polarizationdependence of the device.
 20. The method of claim 19 above, wherein theoptical waveguide device includes a merged optical fiber waist regionincluding a Bragg grating and wherein the step of reducing thewavelength differential comprises shaping the cross-section of the waistregion during fabrication to cause the spaced apart wavelengths tosuperimpose.
 21. The method of claim 19 above, wherein the opticalwaveguide device includes a merged optical fiber waist region includinga Bragg grating and wherein the step of reducing the wavelengthdifferential comprises angularly deforming the waist region about itslength to establish a third intermediate and polarization insensitivemode while separating the wavelengths of the polarized modes andreducing their amplitudes.
 22. The method of equalizing the local modalpropagation constants along a length of optical waveguide that is toinclude a periodic test grating that comprises the steps of: locallymeasuring a center Bragg wavelength of the test grating along the lengthof grating; and locally varying the material index of refraction alongthe length of the test grating to minimize variations in the centerBragg wavelength along the grating to <0.5 nm.
 23. The method as setforth in claim 22 above, further including the steps of writing a testgrating in the optical waveguide, while photosensitizing the opticalwaveguide by in-diffusing a photosensitizing constituent into thewaveguide during the writing of the test grating.
 24. The method as setforth in claim 22 above, wherein the step of locally measuring thecenter Bragg wavelength of the test grating comprises transmittingoptical waves into the waveguide and analyzing position andwavelength-varying reflections therefrom.
 25. The method as set forth inclaim 22 above, wherein the local measurement provides a reading that isrepresentative of the local cross-sectional dimensional variations ofthe waveguide and wherein the step of locally varying the material indexof refraction comprises scanning the waveguide with an ultraviolet beam.26. The method of providing a narrow bandwidth grating in an opticalwaveguide device having a merged narrow diameter section reduced todimensions in which the waveguide core substantially vanishes to avestigial core which is surrounded by original cladding material andsmall dimensional variations affect the modal index of refraction,comprising the steps of: writing a grating in the merged section of thedevice; and measuring a local Bragg wavelength of the grating along themerged section.